1. Field of the Invention
The present invention relates to apparatus for producing a modulated signal from an optical motion picture sound track. More specifically, the invention relates to sound reproduction apparatus that is responsive to density variations of an optical dye sound track.
2. Description of the Prior Art
Optical motion picture sound tracks generally come in one of two forms, either variable-density or variable-area (width). As shown in FIG. 1, a variable-density track image 1 on a motion picture film 2 consists of a constant-width strip the density of which varies from dark to light as a function of the sound recorded. It is characterized by bars of varying density in narrow lateral lines along the edge of the film.
As shown in FIG. 2, a variable-area track image 3 consists of a transparent strip whose width W (and, hence, area) varies as a function of the volume and frequency of sound recorded. It is characterized by one or more narrow, irregular, very dense patterns along the edge of the film. The rate of spatial repetition of the irregular edges determines the frequency of the recorded sound while the width of the transparent portion determines the volume of the recorded sound. For optimum quality on a variable-area sound track image, the clear portions should be as transparent as possible, and the dark portions should be nearly opaque, with a density between 1.0 and 1.5.
The present invention is equally applicable to either type of sound track image. However, to simplify the disclosure, the description will be with reference to variable-area sound track images only.
Turning again to FIG. 2, the basic elements are shown for reproducing sound information from an optical sound track image. Illumination from a lamp 4 is restricted by a narrow slit 5 so that an image 6 of the slit is produced on the film by an objective lens 7. A photoreceptor 8 is placed on the opposite side of the film to receive light transmitted through the transparent areas in the track. Since the width of the transparent area of the track 3 is a function of the recorded sound, the illumination reaching the photoreceptor 8 is likewise a function of the sound signal used to generate the density variations in the track image. For that reason the light striking the photoreceptor 8 is said to be modulated by the optical sound track image.
The effectiveness with which a photographic sound track is reproduced is a function of the spectral energy distribution of the illuminant, the spectral absorption of the sound track image, and the spectral response of the photoreceptor. Illumination is usually provided by a comparatively low color temperature tungsten sound lamp that radiates much more strongly in the infra-red than in the visible spectrum. A typical photoreceptor incorporates a silicon substrate material that is preferentially responsive to radiation in the infrared region. Therefore it is desirable that sound track images have significant, if not peak, absorption in the red to infrared spectral region. Such sensitivity would make the spectral absorption (i.e., modulation) of the sound track material correspond to the spectral emission of available illuminates and the spectral region of greatest receptivity of the conventional receptor. However, a useful material for forming the sound track density image is the color dye used in the production of the color image on the motion picture film. The great majority of such dyes absorb radiation predominantly in the visible region of the spectrum, becoming relatively transparent in the near infrared region and beyond. Consequently, such color dyes do not sufficiently absorb the infrared emission of the sound lamp to adequately modulate the transmitted infrared light. Moreover, the unmodulated infrared component that passes through the sound track excites the conventional photoreceptor to produce an undesirable noise component in its output signal when used in conjunction with color dye sound track images.
Referring to FIGS. 3 and 4, the disadvantages of conventional sound photoreceptors can be appreciated through comparison of the spectral dye density curve 10 (FIG. 3) of a typical motion picture color print film with the spectral response 12 (FIG. 4) of a typical silicon photoreceptor to tungsten illumination. The curve 10 represents the additive combination of the spectral dye densities of the three component dye layers of a typical color film, e.g., the cyan-forming layer 14, the magenta-forming layer 16 and the yellow-forming layer 18. For ease of illustration and explanation, the composite spectral dye density curve 10 will be hereafter used to describe absorption in a conventional optical dye sound track. As FIG. 3 shows, peak modulation occurs in the visible region of the spectrum where most of the density is generated, i.e., from wavelengths of about 400 to approximately 700 nanometers. Furthermore, the track possesses very little density to radiation of wavelengths longer than about 700 nanometers, e.g., infrared radiation. This means that infrared light directed through the film track is substantially unmodulated by density in the track, and contributes to overall noise in the sound reproduction system.
On the other hand, FIG. 4 shows that the spectral response 12 (to tungsten illumination) of a typical silicon material used in a sound photoreceptor peaks at a wavelength of about 800 nanometers, i.e., in the infrared region of the spectrum. This means that the photoreceptor most efficiently responds to light modulated toward the infrared region of the spectrum. But, as described above, the sound track does not effectively modulate light in this region. This inconsistency has been a problem in motion picture sound systems since the introduction of color motion picture film.
To broaden the spectral absorption of the sound track while controlling the noise problem, sound systems of the prior art have utilized coatings on the optical sound track which are responsive to infrared energy. For example, sound track images have been produced with silver or silver compounds, either by a completely separate redevelopment step, or by means of edge treatment of the film at some stage of the processing. The film is passed through a sound-track applicator that deposits a viscous developer or a sulfide solution on the sound track to produce a metallic silver or silver sulfide image. The boundary of the area of action of the developer fluid must be confined to a zone corresponding to the width of the optical sound track, and the action of the fluid must be uniform across the zone of application. Since the developer employed is relatively potent, any spread of the developer into the image area will be immediately destructive of the image. Thus the sound-track applicator requires careful design and precision fabrication while careful control of the viscosity, temperature, and moisture content of the partially developed film is necessary for successful redevelopment. The redevelopment process provides color print films with an optical sound track image consisting of a metallic silver image corresponding to the dye image which still remains in the track. Since the silver image has a relatively uniform spectral absorption through the visible and near-infrared regions, the silver is the effective part of the track, and the dye contributes relatively little. Reversal color films have a silver track image without dye, or in some cases, a track consisting of silver sulfide, which is a reasonably good infrared absorber.
Due to the extremely small dimensions of the sound track in small film formats, e.g., 16 mm and 8 mm, redevelopment is technically complex and can result in a high proportion of losses. To avoid redevelopment and adapt the specral sensitivity of the photoreceptor to the spectral region of greatest sound modulation, a photoreceptor can be used which has sensitivity only in the visible region, where the dye track most effectively modulates the light. This has been indirectly accomplished in the prior art by confining the response of a conventional photoreceptor to the visible portion of the spectrum. For example, Super 8 projectors adapted for use with dye sound tracks have been provided with a photoreceptor having a filter that blocks the infrared component of the impinging light. The photoreceptor, although still inherently responsive in the infrared, is thus confined to "see" only visible light modulated by the dye sound track image. However, the preferred silicon photoreceptors have a much lower response in the visible region compared to their peak response in the infrared region of the spectrum. Therefore the signal-to-noise figure of the filtered photoreceptor is less than the device's inherent capability. Thus the use of dye tracks ordinarily represents some compromise of sound quality compared to the use of silver or silver sulfide tracks with a given type of receptor.
Since the sound modulation signal is mainly determined by the absorption characteristics of the track material in the spectral region in which the photoelectric cell is sensitive, it may at first appear worthwhile to use a dye material, in at least one of the film layers, having peak absorption in the infrared region. While this has been considered from time to time, it does not appear beneficial as a general solution since the choice of dye material would necessarily be limited by sound track considerations, rather than by considerations of the photographic image.
Thus with present sound reproduction techniques there is a choice between, on the one hand, a technically exacting redevelopment process for restructuring the dye sound track or, on the other hand, a dye-based system incorporating a photoreceptor confined to respond to radiation in a less preferential response region.